This invention relates to a circuit for enabling the correction of the phase of a color signal in a video processing system, and particularly relates to a circuit and method for inserting chroma phase information in a video data path and for subsequently recovering the chroma phase information downstream.
Presently known video signal processing systems generally employ a main memory for storing video signals and a timing correction circuit for correcting the time base or frequency errors that occur during the processing of the video signal. Such errors in timing or frequency degrade the video signal and the resultant television image.
Another problem that is experiences with video signal processing systems that incorporate relatively complex circuitry is color instability that adversely affects the color representation of the television image.
In the United States, television signal processing systems employ a standard NTSC format wherein each successive horizontal line is characterized by subcarrier signals of opposite phase, so that there are two different line types when using such a format. With the NTSC system, it is necessary that the chroma signal appearing at the output of the videotape recorder be coincident with the phase of the station reference signal, in order to ensure the correct color of the television picture. The television recording systems employ a time base correction network, which includes a chroma processor for correcting color and a dropout compensator to compensate for missing video signals. The time base correction system requires the identification of the incoming subcarrier phase, as well as the output subcarrier phase which then are compared in order to provide proper color correction and the correct color in the resultant television picture.
In countries that use a PAL standard format which employs horizontal lines of four different subcarrier phases resulting in four different line types, identification of each of the line types is necessary for proper color correction.
In prior art systems, circuits for identifying the chroma line type are generally located in a stage or stages preceding the main memory and the time base corrector (TBC) circuit. However, it has been found that with recently developed TBC systems, which are substantial improvements over prior art systems, color correction effectively is accomplished when processing the video signal utilizing the chroma line type information after time base correction is achieved. It would be desirable to accomplish the chroma phase correction after time base correction and with minimal circuitry.
The present invention overcomes the various disadvantages of previous mention in a time base correction system which includes chroma phase correction, wherein color picture stabilization is achieved after the time base correction error has been removed. To this 3end, the invention provides means for carrying the required line type identifying signals through the main time base correction process of the main memory for use on the reference side of the TBC, utilizing a minimal amount of circuitry while eliminating the need for the overhead of additional data bits.
In a television VTR playback system made in accordance with this invention, color correction is achieved by encoding chroma phase as line type information in the form of a digital word at the beginning of the active picture period of each horizontal line of incoming video data and timebase prior to the processing of the video data via the main memory. The chroma line type identified by the digital word is passed to the output of the memory and is extracted to identify each line, whereby phase correction of the chroma signal is accomplished after time base correction in the same channel.